Reactivity, Regioselectivity, and Synthetic Application of 2-Pyrenyl Units in Scholl Reactions

We herein report the reactivity and regioselectivity of 2-pyrenyl as a coupling unit in Scholl reactions. On the basis of the Scholl reactions of hexaarylbenzene substrates, we have found that pyrenyl units are preferably oxidized over naphthyl and phenyl units under appropriate Scholl reaction conditions, allowing divergent synthesis through a highly controllable intramolecular coupling sequence. We find that the C1 and C3 positions of the 2-pyrenyl unit are the favorable sites for intramolecular coupling while C4 is not reactive to allow further coupling. The reactivity and regioselectivity pattern can be explained by the spin density distribution, which shows that carbon-carbon bonds preferably form at sites with higher positive spin density. Guided by these findings, we successfully synthesized a double helicene and a sextuple helicene through the controlled Scholl reactions of 2-pyrenyl units.